Atomic layer deposition of platinum clusters on titania
nanoparticles at atmospheric pressure
Aristeidis Goulas and J. Ruud van Ommen*
We report the fabrication of platinum nanoclusters with a narrow size distribution on TiO2nanoparticles using atomic layer deposition. With MeCpPtMe3 and ozone as reactants, the deposition can be carried out at a relatively low temperature of 250C. Our approach of working with suspended nanoparticles at atmospheric pressure gives precise control of the material properties, high efficiency of the use of the platinum precursor, and the possibility for large-scale production of the nanostructured particles.
Platinum (Pt) is widely applied to catalyse chemical reactions, despite its high price. Pt-based nanostructured catalysts are for example used in (petro)chemical and rening processes (hydro-, dehydrogenation reactions), fuel cells, automotive applications (emission control, combustion promotion) andne chemical production (bioconversion, selective oxidation).1–5 For opti-mizing the catalyst performance – its activity, selectivity, and stability– while keeping the amount of scarce and expensive utilized Pt at a minimum, it is crucial to maintain excellent control over the morphology and distribution of the metal clusters during their synthesis process. Traditional Pt nano-particle deposition techniques, such as impregnation,6,7 deposition–precipitation,7,8 and ion-exchange,9,10 rely on wet-chemistry schemes and oen result in inefficient control of the active phase growth. In the past years, atomic layer deposition (ALD) has appeared as an appealing nanofabrication technique for the preparation of precisely tailored heterogeneous catalysts.11,12
In ALD, two self-limiting and complementary reactions are used in an alternating sequence to build-up solid lms on a surface. Due to thenite initial number of surface sites, the reactions can only deposit a certain number of surface species.12,13The majority of ALD research is aimed at depositing continuous lms for microelectronic applications. Due to its high surface (cohesive) energy (2.5 J m 2) Pt deposition on
oxide supports such as titania proceeds via an island growth mechanism (Volmer–Weber mechanism) during the initial stages of ALD processes.14–16 Ultimately, aer a sufficient number of exposure cycles, the deposited islands will merge to form a Pt thinlm. However, for applications in catalysis it is typically undesirable to obtain a continuous lm: the island structure should be maintained to maximize the dispersion. Elam et al.17 and King et al.18 described the nucleation and growth of Pt on oxide support materials referring to the potential benets for catalysis applications.
For most catalytic applications, it is crucial to deposit the catalytic material on a particulate support material. To the best of our knowledge, Lashdaf et al.19were therst to report ALD to deposit Pt on a powder support (silica and alumina) for the fabrication of a catalyst. They used aow-type reactor, in which an inert gas containing the precursors was owing along the substrate at a reduced pressure. Such aow-type reactor is the most commonly used device to carry-out ALD.20,21 Flow-type reactors are usually designed for conducting deposition onat substrates and therefore are not optimized for use with powder support materials; they can only handle amounts up to a few grams. For real-life catalytic applications it is crucial that large amounts of Pt-containing catalysts can be synthesized while maintaining properties such as homogeneous distribution and a monodispersed size of the Pt clusters. Following the intro-duction of the use of auidized bed reactor for ALD by Wank et al.22several efforts have been made towards the application of uidized beds as efficient ALD reactors for powder coating applications.23,24Recently, Weimer et al.25–27demonstrated the use ofuidized bed reactors operated under reduced pressure to deposit Pt on micron- and nano-sized powder supports using an organometallic Pt complex and oxygen. In theuidized-bed ALD reactor, the inert gas ow carrying the reactants travels through the collection of particles at such a velocity that they are suspended. Highly dispersed Pt islands were deposited although there was some decomposition of the precursor.26,27A disadvantage of operating a uidized bed at low pressure is, however, that the mixing is strongly reduced; especially when
Del University of Technology, Department of Chemical Engineering, 2628 BL Del, The Netherlands. E-mail: j.r.vanommen@tudel.nl
Cite this:J. Mater. Chem. A, 2013, 1, 4647
Received 28th December 2012 Accepted 21st February 2013 DOI: 10.1039/c3ta01665j www.rsc.org/MaterialsA
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scaling-up the system this will negatively inuence the product properties.28,29
In this communication we present a novel approach to deposit Pt on nanoparticles in auidized-bed ALD reactor at atmospheric pressure, using an ozone-containing air stream as an oxidizer medium. So far, ALD deposition of Pt has only been shown at vacuum. We will demonstrate that the precision characteristics of ALD can be maintained at atmospheric pres-sure, effectively proving the potential of the process for catalyst fabrication on an industrially relevant scale.
The experiments were carried out in a custom-build uidized-bed ALD reactor operated at atmospheric pressure. The noble metal precursor, (trimethyl)methylcyclopentadienyl-platinum(IV) (MeCpPtMe3, 99% purity) was obtained from Strem Chemicals and was used as received. It was contained in a heated (50C) steel vessel connected to the reactor. Air enriched with ozone (around 1.5 wt%) was used as an oxidizer medium; this stream was obtained by leading an air ow (0.20 L min 1) through an OAS Topzone ozone generator. We checked that the reactor feeding system did not lead to signicant ozone decomposition. Aeroxide P-25 titanium dioxide particles (TiO2, $99.5% purity) from Evonik were used as the substrate for the deposition. They have an average primary particle diameter of 21 nm and a surface area of 50 m2 g 1 (determined in a Quantachrome Autosorb-6B). The crystal structure is about 85% anatase and 15% rutile. The reactor chamber consists of a vertical glass tube with an inner diameter of 10 mm, which is lled with 0.25 g of particles. At the bottom of the glass tube, a porous metal plate is placed to contain the particles, allowing an upward gasow. An inert gas ow of dry nitrogen (N2, 99.999 vol %) is provided in the upward direction to suspend the particles. By setting the gasow to 0.20 L min 1(ow rate corresponding to a supercial gas velocity of 4.2 cm s 1) we ensured that the particles were well suspended (uidized) while preventing them from being blown out from the column. The column was placed on a vibration table to ensure properuidization of the cohesive powder. An infrared-heater with feedback control was used to keep the reactor at the desired reaction temperature of 250C. A more elaborate description of a similar setup is given by Beetstra et al.23Aer loading the particles in the reactor, a pre-treatment procedure (heating under inert gasow) was followed to ensure a constant initial number of OH-groups on the surface of the support. The main ALD process sequence involved alternating pulsing of the two precursors, separated by sufficient purging times of inert gas. A typical pulsing sequence for Pt–N2–O3–N2 consisted of exposure times of 3–10–10–10 min. We determined the amount of Pt deposited on the substrate by inductively coupled plasma optical emission spectrometry (ICP-OES) using a Perkin Elmer Optima 3000 DV optical emission spectrometer. The carbon content of the samples was measured by IR absorp-tion spectroscopy using a carbon/sulphur determinator (LECO CS-225). TEM pictures of the samples were taken using a FEI Tecnai TF20 equipped with an EDX detector (Oxford Instru-ments). XRD measurements were carried out with a Bruker D8 Advance diffractometer.
In ALD processes, an important characteristic is the amount of material deposited as a function of the exposure time and of
the number of cycles applied. During the ALD process, satura-tion should occur when the precursor exposure time is long enough. Because of the total surface area in our experiments (12–14 m2per sample batch), a long precursor exposure time was required, aer about 12 min saturation occurred, and a loading of 1.6 wt% was determined with ICP-OES. In this case, just50% of the Pt provided via the precursor was loaded onto the substrate. However, when aiming for the growth of islands rather than a continuous lm, there is no need to go to complete saturation. We found that aer 3 min we already reached a deposition amount that was about half of the satu-rated deposition amount. Fig. 1 shows the measured wt% of Pt (,) as well as the theoretical wt% predicted for ideal ALD (x). The results show that about 95% of the precursor provided was converted into Pt clusters, which is attractive for an expensive metal such as Pt. When processing larger amounts of powder, it will be possible to reduce even further these losses. Fig. 1 also shows that increasing the number of cycles from 1 to 4, a monotonic increase in the Pt loading was observed. Addition-ally, the surface area (A) showed a small increase (about 5% for 4 cycles, in agreement with previous studies on non-porous supports30) that could be justied by the growth of Pt nano-clusters. This Pt loading of the 1-cycle experiments is about 10% higher than what Zhou et al.26obtained at 400 C, and more
than double of what they obtained at 250C with oxygen as the second reactant. The main reason for that is probably related to our use of ozone, which is a more powerful oxidizer than oxygen (the redox potential of O3is 2.07 V, while for O2it is 1.23 V).
It is attractive to synthesize the Pt nanoclusters at a low temperature, since this prevents their coalescence. We show in this work, that the use of ozone allows us to operate in the lower limit of the temperature window of ALD processing,31,32 achieving more uniform deposition conditions without compromising the rate of material growth. Additionally, future use of high-surface area supports is expected to be favoured by the controlled deposition nature of ALD, resulting in high-dispersion values.25,27
For catalytic applications, dispersion of the Pt nanoclusters is oen of great importance. We investigated this examining several TEM images of the prepared materials. To quantify the particle size and its spread, we applied image analysis to
Fig. 1 Pt loading measured by ICP (,) and predicted for ideal ALD (x) along with changes in specific surface area (A) as a function of ALD cycles (Pt content values corrected for changes in specific surface area).
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multiple TEM images. In order to get good statistics, this was done for samples with higher Pt loadings: 12 min deposition time for 1 ALD cycle (1.6 wt%) and 5 min deposition time for 5 ALD cycles (5.8 wt%). For both loadings about 150 Pt nano-particles were processed. Fig. 2(a) shows the TiO2 support particles aer 1 ALD cycle. A uniform distribution of the active phase Pt particles can be seen. The presence of Pt was veried by qualitative EDX elemental analysis. Fig. 2(b) shows the substrate aer 5 ALD cycles. The average particle size clearly increased for an increase in the Pt content from 1.6 to 5.8 wt%. The carbon content of all the samples was very low (less than 0.5 wt%) as obtained by IR absorption spectroscopy.
Fig. 3 gives the particle size distribution for both cases. The particle diameter aer 1 cycle ALD is just 1.5 nm, with a very narrow size distribution. Increasing the Pt loading with a higher number of cycles increases the particle size to 2.3 nm, leading to a less narrow size distribution. These results further indicate that ALD is a precise fabrication technique for both low- and medium-loading noble metal catalysts.
ALD-tailored nanostructured materials show very promising potential for noble metal heterogeneous catalyst applications wherenely distributed ultra-dispersed particles can result in signicant cost reductions: when so well dispersed, the catalytic material is used much more efficiently. Moreover, the demon-strated method is efficient in its use of platinum. The operation in atmospheric pressure conditions strongly enhances the potential for scale-up of the process: it makes the handling and mixing of the powder that serves as a substrate much easier.
Conclusions
In summary, we demonstrated that small Pt nanoclusters of controlled size can be deposited on TiO2nanoparticles (21 nm) by ALD in a uidized bed at atmospheric pressure, using MeCpPtMe3 and ozone as reactants. The use of ozone as an oxidizer enabled us to obtain successful ALD at a temperature as low as 250 C. By using relatively short cycle times to avoid reaching saturation, we were able to utilize the Pt-precursor with 95% efficiency. By varying the number of ALD cycles we were able to precisely control the catalyst loading obtaining homogeneously distributed, highly dispersed Pt nanoclusters with a narrow particle size distribution. This demonstrates the potential of the approach to produce high-quality catalysts, while the operation at atmospheric pressure enables the synthesis of large amounts of material.
Notes and references
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Fig. 2 TEM image of Pt nanoclusters deposited onto the TiO2support after
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Fig. 3 Particle size distribution from image analysis of TEM pictures for Pt nanoclusters deposited onto the TiO2support after 1 ALD cycle (a) and after
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